Proceedings of ICRC 2001: 3943 c Copernicus Gesellschaft 2001

ICRC 2001

Observation of cosmic ray sidereal time variation by GRAPES III muon
telescope at Ooty

H.Kojima3, K.Fujimoto3, S.K.Gupta2, Y.Hayashi 1, N.Ito1, AJain2,S.Kawakami1, D.K.Mohanty2,T.Nonaka1,S.Noto1,A.K.C.Ravindran2, K.Satomi1,K.Sivaprasad2, H.Tanaka 1, T.Toyofuku1,S.C.Tonwar2 and T.Yoshikoshi 1
1
Department of Physics, Osaka City University, Osaka 558-8585, Japan2
TataInstitute of Fundamental Research, Mumbai 400 005, India3Nagoya Womens
University ,Nagoya 467-8610,Japan
H. Kojima1 , K. Fujimoto1 , S. K. Gupta2 , Y. Hayashi3 , N. Ito3 , A. Jain2 , S. Kawakami3 , D. K. Mohanty2 , T. Nonaka3 , S.
Noto3 , K. C. Ravindran2 , K. Satomi3 , K. Sivaprasad2 , H. Tanaka3 , T. Toyofuku3 , S. C. Tonwar2 , and T. Yoshikoshi3

Observation of cosmic ray sidereal time variation by GRAPES III
muon telescope at Ooty
1Nagoya Womens University ,Nagoya 467-8610,JapanTata Institute of Fundamental Research, Mumbai 400 005, India 3
3Department of Physics, Osaka City University, Osaka 558-8585, Japan2Abstract. We analyzed the sidereal time variation on the data ofmuons counting rate observed by the large muon telescope ofGRAPES III ( total area 560 m2 , muon’s energy > 1GeV) over 3
years at Ooty ( 11.4deg latitude, 76.7deg longitude). Theircounting rate is around 53,000 counts /sec and this high counting
rate is great advantage for modulation measurement. The analysis
based on the data of such high-statistics enable us to compare thesidereal diurnal variation even with each single year’s result. We
observed the Tail-in and Loss corn anisotropies through detailed
analysis. We newly started the measurement of direction ofindividual muons with accuracy of about 8 degrees from 1999.
We present some results on the sidereal daily variation obtained bythis system too.
1. Introduction
There are several reports on sidereal time variation of the cosmic
ray, 1010eV~1013eV, so far. Swinson(1969) analyzed the intensity
variation of the cosmic rays with energy 1011eV-1012eV and hereported that there are some days which have the maximum peakat local sidereal time 18 and other days have it at 6 hour. These
two different timing of peak position is due to polarity change(Tword, Away ) of interplanetary magnetic field. This is basedon the flow of the cosmic ray which is perpendicular to ecliptic
plane and existence of the radial density gradient of cosmic ray ininterplanetary magnetic field near Earth. Thus these sidereal timeanisotropy comes from local helio-magnetosphere.
Many other groups also reported the possible existence of thecosmic ray anisotropy caused by outside helio-magnetosphere.
Alexeenko and Navara(1985) proposed the existence of theanisotropy by the flow of the high energy diffusion gamma-rays
which are confined in our galactic disk, from the result of the air
shower experiment (<1013eV) in Baksan. They insisted suchanisotropy should make the peak at 0 hour local sidereal time innorthern hemisphere. If there is such gamma rays flow, there
should be peak at 12 hour local time in southern hemisphere.
On the other hand, Nagashima and Fujimoto et al(1989) analyzed
the data of air shower direction (>1013eV) in Mt. Norikura, andhave shown the minimum intensity occurs at local sidereal time of
12 hour. They proposed that the minimum intensity occurs at 12hour local sidereal time in the northern hemisphere rather thanmaximum occur at 0 hour which is shown by Alexeenko et al.
Considering there is no peak in the southern hemisphere atsidereal time of 12 hour, they proposed the following model: theintensity loss comes from loss cone which is created by theinteraction between cosmic rays and magnetic field of theinterstellar space. In addition to the Mt Norikuraata's air shower
data, Nagashima Fujimoto and Jacklyn (1998) analyzed the dataon Hobart and Sakashita's underground muon telescope
While the solar system moves in our galaxy, thehelio-magnetosphere has the bow shock structure.
The cosmic rays are easy to infiltrate from tail direction of thisstructure.
This lead to another anisotropy which shows maximum at 6 hour
local sidereal time for the energy around 1012 eV of cosmic rays
entering from southern hemisphere. These two anisotropy are
respectively called loss cone (LC) and tail in (TI).
The GRAPES III muon telescope is situated in Ooty (11.4 deg.
latitude, 76.7deg. longitude) and the cosmic ray of the energy
above 1011eV is observed . This telescope is very much suitable toobserve sidereal time anisotropy. We are hoping to clarify thecoordinate of excess and deficiency of cosmic rays due to LC andTI more clearly by adding the recent results which are obtained byangle measurement of muons.

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2. Analysis and result
We analyzed the data of the muon observed by the GRAPES
III muon telescope and obtained the sidereal time anisotropy ofthe primary cosmic ray, the energy about 1011eV from. (total areaof 560 m2, muon energy > 1GeV). The observation period omni
directional observation is 3 years from April, 1998 to March,
2001 .
Total counting rate is 53,000/sec. for omni-directional
measurement with all the modules. This high counting rate assures
us very small statistical error (0.02% with 10min. data).
The process of our basic analysis is the followings
1. Data are analyzed in terms of one month after correcting
barometric pressure and normalized by one month average.
2. This monthly data is categorized into two groups with theirpolarity of Interplanetary Magnetic Field (IMF). This polarity
is calculated by the following method. We calculated theaveraged IMF value at UT 20 hour for different polarity. It isobtained by getting 72 hour running average of one hour
value of GSM coordinate, using level-2 data of ACE
SCIENCE CENTER.
3. We make two separate dataset on sidereal time variation, oneis for ‘toward ’ polarity and other is ‘away’.
4. Final results are obtained by making mean value of these twosets .
One of the reason that we use the average of these monthly dataset
is due to large fluctuation and unbalance in number of days ofdifferent polarity. Another reason is that there is rather big Solaranisotropy (with amplitude around 0.4%) and these seasonalchange may cause significant distortion on the sidereal daily
variation. So we estimate that the mean value from the monthly
dataset would be better than the simple darly mean. There seems
to be systematic another seasonal effect which arises from thestationery solar time anisotropy. To avoid this, anti-siderial timecorrection were done. (Nagasima et al,1983) Those results areshown in Fig 1.
Omni-directional measurement

%
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8away

12
toward1620(A+T)/2Fig 1 Siderial time anisotropy of the omni-directional
measurement at Ooty

24

IMF polarity dependence of the sidereal anisotropy is clearly seenfrom these results. This inversion is indicated by Swinson(1969).
This anisotropy which seems to be the Tail In (TI) effect, proposed
by Nagashima, Fujimoto and Jacklyn( 1998), is observed from theaveraging of toward and away.
However, the maximum intensity occurs about 2 hours earlier
than expected timing. This can be well explained, if we consider
the deflection of the cosmic rays by earth’s magnetism. Though
the Loss Cone (LC) anisotropy can not be clearly confirmed, thismight be due to the difference in direction or energy of the
primary cosmic rays.
We have started the angle measurement on individual muon fromMarch 1999 with one module of 35m2 area and 12 modules(420m2) are operational now. The analysis was carried out for one
module for 2 years data , April 1999 to March 2001. Though ourmeasuring system has an accuracy of 7 to 8 degrees in order togain enough statistics we have summed some of the data andarranged in six directions of North East (NE), North West (NW),East(E), West(W), SE and SW. Finally they are further added into
three groups of northern, zenithal and southern direction. The
difference in E and W was taken on each group every hour. using
this differential method, the systematic errors such as
characteristics of the equipment, temperature effect and evenbarometric pressure effect can be removed without furthercorrection. We constructed the sidereal daily variation for thesethree groups. We have analyzed these directional data following
same manor as omni directional data. The results are shown in
Fig2.
As it can be seen clearly in fig.2 in each groups results, themaximum phase occur difference of around 12 hours in case of
Toward and Away.
If we consider that this results came from only one module andobservation time of 2 years and compare with omni directional
observation, it is quite satisfactory.
There are some difference in amplitude of averages value (T+A)/2
among the three groups. Though it is not so significant to say , ourresults are indicating the intensity asymmetry due to L.C. and T.I.
anisotropy, the tendency of revealing these anisotropy can be seen.
Since the analysis of directional muon observation is carried out
only for one module so far we could not achieve sufficient
statistical accuracy yet. But we are operating 12 modules atpresent, we are hoping expected that the structure of L.C. andT.I. anisotropy can be clarified in detail within one to two yeartime.

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ACKOWLEDGMENTS
We are grateful to the Ministry of education of Japan for their
partial financial support for this experiment. We used the IMF
data of ACE SCIENCE CENTER . The authors wish to
thanks Prof. V.S.Narasimham and other member of theTIFR-OCU Proton Decay Collaboration for the loan of
proportional counters used in the muon detector. We are also
happy to acknowledge valuable contributions of G.Paul Francis,K.C.Ravindran, B.Sreenivasa Rao and V.Viswanathan during
the installation, operation and maintenance of theinstrumentation for the GRAPES III array. The help andcooperation of the Radio Astronomy Center for providing sitefacilities for the GRAPES III array are gratefully acknowledged

North%
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TOWARD

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（A+T)/2Zenith
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References
Swinson,D.B, J. Geophys. Res.,74,5591, 1969
Nagashima, K.,et al., Nuovo Cimento,C(6),550. 1983aAlexeenko, V.V., and Navara, G., Lett. Nuovo Cimento, 42(7),321., 1985
Nagashima, K. and Fujimoto, K.,et al., Nuovo Cimento, 12C(6),695., 1989
Nagashima, K., Fujimoto, K., and Jacklyn, R.M., J. Geophys.
Res., 103, A8, 17429., 1998
Munakata, K.,et al., Proc 26th Int. Cosmic Ray Conf., Salt LakeCity, 7, 293-296., 1999

0.05
0-0.05-0.1-0.15
0246810AWAY

1214TOWARD

1618（A+T)/2South%
0.15
0.1
0.05
0-0.05-0.1-0.15
0246810AWAY1214TOWARD

1618（A+T)/2(A+T)/2%0.060.04
0.02
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-0.06
02

468

North

1012

14Zenith

1618SouthFig 2
Sidereal time anisotropy of the directional telescopes(north, zenith, south) and (A+T) /2 of three
directional compornent at Ooty